Physics of Dust and Sand Storms

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Description on how dust storms are generated and what are the proper means to detect it from space.

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    Physics of Dust and Sand Storms

    Alee K. Obeid

    BSc. Mechanical Engineering (Aeronautics) High Level Post Graduate Aerospace Engineering Student

    1. Introduction The wind-driven emission, transport, and deposition of sand and dust by wind are termed

    Aeolian processes, after the Greek god Aeolus, the keeper of the winds. Aeolian processes occur

    wherever there is a supply of granular material and atmospheric winds of sufficient strength to

    move them. On Earth, this occurs mainly in deserts, on beaches, and in other sparsely vegetated

    areas, such as dry lake beds. The blowing of sand and dust in these regions helps shape the

    surface through the formation of sand dunes and ripples, the erosion of rocks, and the creation

    and transport of soil particles. Moreover, airborne dust particles can be transported thousands of

    kilometers from their source region, thereby affecting weather and climate, ecosystem

    productivity, the hydrological cycle, and various other components of the Earth system.[Ref.(1)].

    But Aeolian processes are not confined to Earth, and also occur on Mars, Venus, and the

    Saturnian moon Titan (Greeley and Iversen 1985). On Mars, dust storms occasionally obscure the

    sun over entire regions of the planet for days at a time, while their smaller cousins, dust devils,

    punctuate the mostly clear daytime skies elsewhere (Balme and Greeley 2006). The surface of

    Mars also hosts extensive fields of barchans, transverse, longitudinal, and star-like dunes, as well

    as other exotic dune shapes that have not been documented on Earth (Bourke et al. 2010). On

    Venus, transverse dunes have been identified by the Magellan orbiter (Weitz et al. 1994), while

    the Cassini orbiter has documented extensive longitudinal sand dunes on Titan (Lorenz et al.

    2006). [Ref. (1)].

    The terms dust and sand usually refer to solid inorganic particles that are derived from

    the weathering of rocks. In the geological sciences, sand is defined as mineral (i.e., rock-derived)

    particles with diameters between 62.5 and 2,000 m, whereas dust is defined as particles with

    diameters smaller than 62.5 m (note that the boundary of 62.5 m differs somewhat between

    particle size classification schemes, see Shao 2008, p. 119). In the atmospheric sciences, dust is

    usually defined as the material that can be readily suspended by wind (e.g., Shao 2008), whereas

    sand is rarely suspended and can thus form sand dunes and ripples, which are collectively termed

    bed forms. [Ref. (1)].

    Dust storms are among the most severe environmental problems in certain regions of the

    World. In where they occur most of the dust in the atmosphere is from Aeolian origin. Estimates

    of the total Aeolian dust from deserts in the atmosphere are about ton/yr (Ning Ai and Karen R. Polenske). Several authors (JungeC 1979): Ganor E, Mamane Y 1982: Morales C 1979)

    have estimated that the Sahara desert alone contributes ton/yr or between 4066% of the total dust. Dust storms may be traced as far as 4000 km from their origin. [Ref. (2)].

    Dust storms may cause a variety of problems. One of the major problems is a

    considerable reduction of visibility that limits various activities, increases traffic accidents, and

    may increase the occurrence of vertigo in aircraft pilots (Morales C., 1979; Hagen L.J, Woodruff

    N.P. 1973; Middleton N.J, Chaudhary QZ 1988; Dayan U, Heffter J, Miller J, Gutman G 1991;

    Yong-Seung C, 1996). Other environmental impacts, reported in the literature (Hagen L.J,

    Woodruff N.P. 1973; Mitchell J.M. 1971; Fryrear D.W. 1981; Victor R. Squires. 2007; Jauregui

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    E. 1989; Liu C.M. Ou S.S. 1990; Yong-Seung C, Ma-Beong Y. 1996 and Ning Ai and Karen R.

    Polenske) include reduced soil fertility and damage to crops, a reduction of solar radiation and in

    consequence the efficiency of solar devices, damage to telecommunications and mechanical

    systems, dirt, air pollution, increase of respiratory diseases and so on. Three main categories of

    reduced visibility are often used to describe the severity of dust storms.

    Blowing Dust the horizontal visibility (due to Aeolian dust), is less than 11 km. Dust Storm the horizontal visibility is less than 1000 m. Severe Dust Storm the horizontal visibility is less than 200 m. [Ref. (2)].

    In some seasons in certain regions of the Middle East and North Africa and for about

    30% of the time on average, the dust conditions in the lower troposphere fall into one of these

    three categories. Thus, in these regions, dust storms are a very frequent phenomenon and a better

    knowledge of their spatial and temporal distribution is of prime importance. A positive

    correlation exists between the quantity of dust in the air, and the wind velocity. Whereas, a

    negative correlation exists between dust amount and the particles size. Precipitation and/or vegetation coverage may reduce considerably the amount of dust in the air for a given wind

    velocity and/or particles size (Bagnold R.A. 1941; Gillette D.A. 1979; Mitchell J.M. 1971). Thus, a study of the atmospheric circulation and its impact on the precipitation regime in a given

    region is crucial to understand the dust distribution in that region. [Ref. (2)].

    A previous attempt to delimit the regions in the Middle-East according to the seasons of

    main activity was done by Middleton (Middleton N.J. 1988). He analyzed the dust distribution

    over Syria, Lebanon, Jordan, Israel, Saudi Arabia, Yemen, Iraq and Iran and his analyses were

    based on short periods of data recording and on other data collected over varying lengths of time

    from the 1950s and 1960s. The present study extends Middletons analysis in three ways: (a) the study area was extended to include also data from Turkey, Cyprus, Egypt and Sudan, (b) the

    analysis period was extended to 21 years (19731993) and (c) the clustering of the different stations into coherent regions was done in an objective way using cluster analysis. [Ref. (2)].

    1.1 Manners of Wind-Blown particle transport

    The transport of particles by wind can occur in several manners, which depend

    predominantly on particle size and wind speed (Figure 1.1). As wind speed increases, sand

    particles of ~100 m diameter are the first to be moved by fluid drag. After lifting, these particles

    hop along the surface in a process known as saltation (Bagnold 1941, Shao 2008), from the Latin

    salto, which means to leap or spring. The impact of these saltators on the soil surface can

    mobilize particles of a wide range of sizes. Indeed, dust particles are not normally directly lifted

    by wind because their interparticle cohesive forces are large compared to aerodynamic forces.

    Instead, these small particles are predominantly ejected from the soil by the impacts of saltating

    particles (Gillette et al. 1974, Shao et al. 1993a). Following ejection, dust particles are susceptible

    to turbulent fluctuations and thus usually enter short-term (~ 20 - 70 m diameter) or long-term

    (< ~20 m diameter) suspension (Figure 1.1). Long-term suspended dust can remain in the

    atmosphere up to several weeks and can thus be transported thousands of kilometers from source

    regions (Gillette and Walker 1977, Zender et al. 2003a, Miller et al. 2006). These dust aerosols

    affect the Earth and Mars systems through a wide variety of interactions. [Ref.(1)].

    The impacts of saltating particles can also mobilize larger particles. However, the

    acceleration of particles with diameters in excess of ~500 m is strongly limited by their large

    inertia, and these particles generally do not saltate (Shao, 2008). Instead, they usually settle back

    to the soil after a short hop of generally less than a centimeter, in a manner of transport known as

    reptation (Ungar and Haff 1987). Alternatively, larger particles can roll or slide along the surface,

    driven by impacts of saltating particles and wind drag forces in a mode of transport known as

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    creep (Bagnold 1937). Creep and reptation can account for a substantial fraction of the total wind-

    blown sand flux (Bagnold 1937, Namikas 2003). [Ref.(1)].

    The transport of soil particles by wind can thus be crudely separated into several physical

    regimes: long-term suspension (< ~20 m diameter), short-term suspension (~20 70 m), saltation (~70 500 m), and reptation and creep (> ~500 m) (Figure 1.1). Note that these four transport modes are not discrete: each mode morphs continuously into the next with changing

    wind speed, particle size, and soil size distribution. The divisions based on particle size between

    these regimes are thus merely approximate. [Ref.(1)].

    Figure 1.1. Schematic of the different modes of Aeolian transport. [Ref.(1)].

    A recent study finds that the initial saltation of sand particles induces a static electric field

    by friction. Saltating sand acquires a negative charge relative to the ground which in turn loosens

    more sand particles which then begin saltating. This process has been found to double the number

    of particles predicted by previous theories. (Electric Sand Findings 2008). [Ref. (2)].

    1.2 Contribution of study of wind-blown sand and dust to the Earth and planetary sciences

    Wind-blown sand has shaped a substantial portion of the Earths surface by creating sand dunes and ripples in both coastal and arid regions (Bagnold 1941, Pye and Tsoar 1990), and by

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